Digital communication over wired transmission media is achieved by launching electromagnetic signals onto the wires at one end and capturing them at the other end of transmission lines. Wired transmission media include metallic wires, pairs or sets of metallic wires used together, coaxial cables, and even optical fiber. A point-to-point communication link over a wired transmission line can support two-way communications using transceivers at each end of the wire that feature both transmit and receive capabilities. Full-duplex modems support simultaneous transmit and receive capabilities by applying electronic and/or modulation techniques to separate the received signal from the transmitted signal.
An illustrative digital subscriber line (xDSL) implementation of two modems 10, 12 communicating over a twisted-pair copper wire transmission line 14 connecting a telephone company central office (CO) to a customer premises is shown in FIG. 1. At the transmission line input to the near end modem, just past line coupling circuitry 15, the total signal is formed by superposition of the signal transmitted from the near end 10 and the signal received from the far end 12. To support simultaneous communication in both directions (full-duplex operation), some method of separating the received signal from the transmitted signal must be employed so that a transmitter 16 and receiver 18 in each modem 10, 12 can operate simultaneously. A popular method incorporates balanced line coupling circuitry to ensure that 100% of the signal to be transmitted is transferred to the transmission line 14 and 0% is reflected back into the receive path, eliminating all interference into the received signal. In the case of twisted-pair copper wire telephone transmission lines, the line coupling circuitry 15 converts the two-wire bidirectional transmission line 14 to a four-wire path with separate unidirectional transmit and receive circuits 21, 22. This line coupling circuit (and converter) is often referred to as a xe2x80x9chybrid.xe2x80x9d See, Whitman D. Reeve, xe2x80x9cSubscriber Loop Signaling and Transmission Handbook: Digitalxe2x80x9d (IEEE Press, Piscataway, N.J. 1995), pages 54-56, incorporated herein by reference. The two-wire transmit and receive paths 21, 22 shown can accommodate differential signals like those conveyed on the phone line or single-ended signals, depending on the hybrid design. For single-ended signals, one wire of the pair conveys the signal and the other provides a ground signal reference.
FIG. 2 shows an electronic hybrid 24 that converts a differential two-wire telephone line 14 to separate single-ended transmit and receive paths 21, 22. (Electronic hybrids featuring differential transmit and receive terminal outputs are very similar, so are not discussed further here.) A transformer 26 provides magnetic inductive coupling of the hybrid circuitry 24 to the telephone line 14. The signal received from the far end passes through transformer 26 to terminal A (FIG. 2), which is connected directly to the positive terminal of a receive amplifier 27. The transmit signal input to transformer terminal A is determined by a voltage divider formed by a resistor R (28) and an impedance Zi, where Zi is the input impedance looking into transformer 26. Zi is a function of the transformer impedance as well as the phone line impedance. To remove the transmit signal from the receive path 22, a cancellation voltage is generated at terminal B by another voltage divider formed by a resistor R (31) and a balance impedance Zb (32). The cancellation voltage is input to the negative terminal of the receive amplifier 27 and only the difference between the negative and positive terminals of the amplifier 27 is passed through as the received signal (common-mode voltages are rejected). See, Paul Horowitz and Winfield Hill, xe2x80x9cThe Art of Electronics: second editionxe2x80x9d (Cambridge University Press, 1989), incorporated herein by reference. If Zb=Zi, then the cancellation voltage exactly matches the transmit voltage and no transmit signal is passed through the receive amplifier 27. This is a complete electronic cancellation.
If Zb is not exactly equal to Zi, an impedance mismatch occurs at line coupling circuitry, 15 and a portion of the transmitted signal, called the echo, will be included in the output of the receive buffer. Any echo that leaks into the receive path 22 will interfere with the attenuated signal from the far end. Characteristics of the echo signal are determined by the specific circuit elements used in the hybrid 24 and the input impedance Zi of transformer 26. The echo can be described by the convolution of the transmitted signal x(t) and h(t), the impulse response describing the echo path through the hybrid 24. The severity of the impedance mismatch determines the magnitude of the echo, which is often defined as echo return loss (ERL), a logarithmic measure of the ratio of power of the transmitted signal to the power of the echo. A very high ERL indicates that very little transmit signal is echoed back, while a low ERL means that the echo is large. The ERL in an xDSL transceiver may be as low as 6 dB in some cases and the signal from the far end may be attenuated by as much as 70 dB. Thus, the received signal may be as much as 64 dB below the echo!
Under controlled conditions, where the hybrid line coupling can be tuned to match the line, the echo can be made arbitrarily small. However, manual tuning is undesirable for cost effective deployment of wireline modems. A better approach is to design the matching circuitry to provide an acceptable level of echo suppression over a wide range of anticipated line conditions. The system should be designed to operate reliably in the presence of echoes produced under the range of anticipated line conditions. This can be achieved with the proper selection and design of modulation techniques and receive circuitry.
Various modulation techniques are available for signal separation. To achieve co-existing transmission and reception in the presence of significant echo due to impedance mismatch in the hybrid circuit 24, a data duplexing method can be incorporated into the modulation technique to help separate the bidirectional data traffic. For example, frequency-division duplexing (FDD) or time-division duplexing (TDD) can be used to separate the outgoing and incoming signals from one another in frequency or time, respectively. However, both FDD and TDD systems sacrifice channel capacity to facilitate signal separation. The idea in both FDD and TDD systems is to consider the transmitted signal as an unwanted and unknown interference into the received signal. The aim of both systems is to remove the unwanted component.
FDD systems place the transmitted signal in a different portion of the frequency spectrum than is occupied by the received signal. The ANSI T1E1.413 specification (ADSL standard) supports FDD operation. See, xe2x80x9cT1.413-1995: Telecommunicationsxe2x80x94Asymmetric Digital Subscriber Line (ADSL) Metallic Interface xe2x80x9d (1995), incorporated herein by reference. Both transmit and receive functions are operated simultaneously, but throughput capacity in either direction is sacrificed because the full bandwidth is not used. TDD techniques separate the outgoing and incoming signals by turning off the transmit portion in order to extract the incoming signal without interference. When transmit mode is entered, the transceiver no longer attempts to receive data. This method has been proposed for the evolving VDSL standard. See, ANSI Contribution T1E1.4/96-329R1, DMT Group VDSL PMD Draft Standard Proposal (February 1997), incorporated herein by reference. Several VDSL proposals refer to the TDD scheme as xe2x80x9cping pongxe2x80x9d because the modems take turns sending the information back and forth. TDD also sacrifices throughput capacity because transmission and reception do not occur simultaneouslyxe2x80x94only a portion of the total time is used for each.
Because the transmitter and receiver are co-located in the same transceiver of a wireline modem, the receiver portion can exploit knowledge of the transmitted signal to extract the reflected portion of it from the receive path. Algorithmic echo cancellation techniques can be applied to first estimate the hybrid echo path, synthesize a cancellation signal based on the transmitted signal and an estimate of the hybrid echo path, and then subtract the cancellation signal from the received signal to alleviate interference. Because the transmitter and receiver operate simultaneously and occupy the full bandwidth for the entire time duration, echo cancellation systems are superior because they provide a higher theoretical capacity for transmitting data.
An example of an echo cancellation system is shown in FIG. 3. A modulator 35 produces transmit symbols that are passed through a digital-to-analog converter (DAC) 37. The DAC output is sent to the electronic hybrid circuit 24 which places the transmitted signal on the phone line 14. The signal received from the far end passes through hybrid 24 and into the input of an analog-to-digital converter (ADC) 39. The echo of the transmitted signal from hybrid 24 is also present at the ADC input. Thus, ADC 39 generates a digital representation of the superimposed signals. To cancel the unwanted echo, the receiver (here, shown as demodulator 40) forms an estimate ĥ[n] of the echo path. This can be done once and saved. The transmitted symbols are convolved with ĥ[n] at 41 to form an estimate of the echo. The echo estimate is subtracted at 42 from the ADC output to produce an echo-free estimate of the received signal that is passed to demodulator 40 for further decoding.
FIG. 4 shows an example of a received signal and an echo signal superimposed at the input of ADC 39. The ADC amplitude rails are set to accommodate the composite signal and a discrete number of available amplitude quantization levels are distributed between the rail values. The composite signal is quantized to the ADC amplitude values. The digital estimate of the echo signal is then subtracted to produce the received signal. However, the received signal is very small compared to the echo signal, so only occupies a small portion of the possible signal range. This results in a larger amount of quantization noise in the received signal than would be present if the received signal occupied the entire range of the ADC input. There is, thus, a need to have an echo cancellation system that has a reduced level of quantization noise.
An example of a hybrid time/frequency based echo cancellation system that operates with a discrete multitone (DMT) receiver of an ADSL modem application is shown in FIG. 5. An encoder 43 produces transmission symbols which are grouped into blocks that are processed with an inverse fast Fourier transform (IFFT) operation (44). A cyclic prefix (CP) is added to the IFFT output at 46 and the result is passed through DAC 37, into hybrid 24, and onto phone line 19. The incoming signal passes from phone line 19 through hybrid 24 and into ADC 39. The total ADC input is the superposition of the received signal and the echo of the transmitted signal that is reflected back through hybrid 24. The ADC output is a digital representation of the superimposed inputs. The ADC output is passed through a time-domain equalizer 48 and the cyclic prefix is removed at 49. A combination of time and frequency domain techniques are used to remove the echo from this signal. A circular echo synthesizer (CES) 50 is used to make the echo appear periodic so that it can be canceled in the frequency domain at 51. See, Richard C. Younce, Peter J. W. Melsa and Samir Kapoor, xe2x80x9cEcho Cancellation for Asymmetrical Digital Subscriber Lines,xe2x80x9d Proceedings of the International Conference on Communications (1994), incorporated herein by reference.
According to a basic Fourier transform property, circular convolution of two sequences in the time-domain is equivalent to complex multiplication of their Fourier transforms. See, Alan V. Oppenheim and Ronald W. Schafer, xe2x80x9cDiscrete-Time Signal Processing,xe2x80x9d (Prentice Hall, N.J., 1989), incorporated herein by reference. Thus, CES 50 serves to make the convolution of the transmitted signal and the cyclic-looking hybrid echo path appear circular. A frequency-domain estimate of the CES-enhanced echo is synthesized by multiplying the frequency-domain representation of the transmitted signal at 52 by a frequency-domain estimate H[k] of the hybrid echo path. The synthesized echo is then subtracted at 53 from the frequency-domain representation of the CES enhanced signal at the output of fast Fourier transform (FFT) operation 54. A more thorough description of frequency-domain techniques can be found in the literature. See, Minnie Ho, John M. Cioffi and John A. C. Bingham, xe2x80x9cHigh-Speed Full-Duplex Echo Cancellation for Discrete Multitone Modulation,xe2x80x9d Proceedings of the International Conference on Communications (1993), incorporated herein by reference.
As described, full-duplex wireline communication between two modems requires techniques for sharing the wire for transmission in both directions. Electronic hybrid circuits in conjunction with digital echo cancellation techniques provide one way to do this. Such techniques suffer, however, from large quantization noise because the echo signal is larger than the received signal at the input to the ADC. To maintain a large analog dynamic range of the received signal at the input to the analog-to-digital converter in the receiver, an analog echo cancellation approach is proposed. The approach utilizes two digital-to-analog conversions in the modem""s analog front end (AFE). One generates the analog signal for transmission. The other generates an analog representation of a cancellation signal that is used to electronically cancel the echo before analog-to-digital conversion of the received signal. To reduce the size, power, and complexity of the modem""s AFE, a preferred embodiment of the invention utilizes multiplexed DAC architecture to emulate two DACs by sharing DAC circuitry between data paths of the two digital-to-analog conversions. Signal processing algorithms are utilized to compensate for differences in output signals of the two DAC paths that result from using a multiplexed DAC architecture. In the frameworks of wireline modems, the additional compensation processing has minimal impact because it can be grouped with other signal processing algorithms in the modem.